You could use small MOSFETs to switch the resistors to a nice quiet analog
ground.  However, parts count is starting to go up quite a bit.  If you are
tempted by a digital pot, I have used the Xicor Quade2pot range quite a bit,
they seem pretty good, although the datasheet is pretty obscure on some
aspects.  As an aside, I have the preliminary datasheet for a new Xicor part
that contains: 3 digital pots (one 64 tap 50k, one 100 tap 10k and one 200
tap 10k), reset supervisor with power on reset, low voltage reset with
adjustable trip levels, a manual reset input, three voltage monitor pins
with user adjustable threshold (one with a hardware output, the other two
readable through the serial bus), 2K eeprom memory and 2.7 to 5.5 volt
operation.  Phew.  Sounds like a very useful part though, it's number is
X6402

The only thing I would check about the LDR arrangement is linearity.  I
don't know how straight the transfer curve of a typical LDR is and combined
with the non-linearity of a typical LED, you would have to use some kind of
lookup table to get specific gains from the circuit.  Certainly do-able
though.

Regards

Mike Rigby-Jones

> -----Original Message-----
> From: Nikolai Golovchenko [SMTP:golovchenko@MAIL.RU]
> Sent: Wednesday, October 27, 1999 7:17 AM
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      Re: Variable gain amp using PIC - possible?
>
> Hello!,
>
>
> Thanks Rob! This editor is what I needed.
> I hope, this drawing is better readable:
>
>        +1.5
>   2.5V -1.5 |\
>    ---------|+ \          RA0
>             |    >----+----
>           +-|- /      |
>           | |/        |
>           |     R     |
>           +---\/\/\---+
>           |
>          --- C
>          ---
>           |
>           |  R1=R    1:2  RA1
>           +---\/\/\---+---
>           |  R2=R/3  1:4  RA2
>           +---\/\/\---+---
>           |  R3=R/7  1:8  RA3
>           +---\/\/\---+---
>           |  R4=R/15 1:16 RA5
>           +---\/\/\---+---
>
> Right, I didn't take into account the noise from current going through PIC
> Besides there is a protection diod on any pin with anode tied to ground!
> The diodes will switch on all resistors when output swings below virtual
> ground (2.5V)  8-(  Because the capacitor starts discharging in that
> condition  )  Will this diod disable any possibility of the plot???
>
> I'm thinking about LDR or digital potentiometers now... Nice thing about
> LDR is that it can be controlled with only one digital line if you use PWM
> - needs only RC network for filtering.
>
> Thanks to all who replied.
> Nikolai
>